Method of coating stainless steel



Patented Jan. 9, 1955i METHOD OF QCOATIN G STAINLESS STEEL Irvine C. Clingan, Baltimore, Md., assignor, by

mesne assignments, to Armco Steel Corporation, a corporation of Ohio N 0 Drawing. Application May 29, 1945, Serial No. 596,593

'7 Claims. (01. lie-cal) l V This application is a continuation-in-part of my copending application, Serial No. 470,853, filed December 31, 1942 and entitled Stainless Steel and Method of Coating Same, now Patent 2,- 394,899 of February 12, 1946, and the invention relates to a process of blackening metallic products, more especially products of stainless steel. Among the objects of my invention is the provision of ,a simple, economical and thoroughly practical process for the blackening of stainless steel or other metal or alloy products, such as machine and equipment parts, implements, trim, finishings, and the like, giving a black coating which is dense and adherent and of high quality,

durability and resistance to corrosion.

Other-objects in part will be obvious and in part pointed out hereinafter.

The invention, accordingly, consists in the several operational steps and the relation of each of the same to-one or more of the others, as described herein, the scope ofthe application of which is indicated in the claims following the description.

As conducive to a clearer understandingof certainfeatures of my invention, it may be noted at this point that stainless steel is defined as a low-carbon steel comprising 10% to 35% chromium, with or Without nickel, and with or without supplemental additions of manganese, silicon,

cobalt, copper, molybdenum, tungsten, vanadium,

columbium, titanium, and the like, for special purposes, and a balance which is substantially all iron.

There is a growing demand for blackened metal products, including those of stainless steel, which differ in appearance from the usual metallic appearance. In meeting this demand, a number of coating or blackening processes of the prior art are employed which in the instance of stain-' less steel afford a finish differing from the unpolished dull gray or briliantly polished natural finish. The more popular coatings among those heretofore obtained are chemical compounds of the metal, which coatings differ from paint or the like in that they aremore adherent, and durable.

The heretofore known chemical processes of black-coating stainless steel have, for the most part, been of limited success, one reason beingthat the steel is characteristically stable under treatment or that the time of processing required is too long for practical purposes. oftentimes, the processes necessarily create dangerous or obnoxious fumes .or other working conditions which are objectionable. Certain of the known proo corrosive effects.

chemicals which deteriorate or are spent in a short while. A considerable expenditure, therefore, usually is entailed in effecting replacement of the treating materials for maintaining standards of treatment.

Among the heretofore known processes are those Whichare wholly incapable of giving satisfactory coatings on stainless steel'products or, as suggested hereinbefore, are not easily controlled to give an expected quality of coating in a consistent manner. Experience with certain methods of blackening stainless steel has shown that some grades blacken less readily than others.

The coatings obtained often lack uniformity from portion to portion of the treated product, fail to meet hardness or toughness requirements or are insufficiently resistant to corrosion. Where unequal thicknesses of a given coating occur, interference colors due to refraction of light through unequal or unduly thin coatings commonly are encountered.

The coatings obtained by certain conventional methods often are porous and as such are inadequatefor protecting the underlying metal from There are also soft coatings as for example the heretofore known hydroxide coatings which are chalky and rub off or wear away too readily to be of practical use. They also undergo change in the presence of moisture.

Other known coatings are not sufiiciently adherent to the underlying metal. Tlfev crack, chip, or spall off, especially where stressingsuch as flexing or binding are encountered. Another class of coatings heretofore achieved on stainless steel as well as on other metals or alloys are thick, and materially affect the final dimensions of finished products, and are to be avoided when close dimensional tolerances are required in production.

An outstanding object of my invention, accordingly, are the provision of a method for producing black-coated stainless steel products, which coating is stable, durable and corrosion resistant under a wide variety of conditionsand even is adherent wl en the metal is bent, which coating is hard and tough and uniform both in texture and appearance, and which, being dense and substantially free of chalkiness, does not rub off or wear away during extended use.

Referring now more particularly to the practice of my invention, I provide stainless steel articles, for example. with black oxide coatings of exceptional quality by immersion treatment of; the steel in an oxidizing bath of molten sodium i a dichromate and/or potassium dichromate while passing oxygen, illustratively in the form of air or commercial oxygen, through the bath, during the period of immersion. The quality and thickness of black coating which I thus obtain on the articles depend upon such factors as time of immersion and temperature of the treating bath. I find that surface condition of the article or roduct when immersion treatment is begun, generally influences the actual appearance of black coating imparted by the bath. A dull gray unpolished stainless steel piece, for example, possesses a dull black appearance, while, a, polished.

piece displays a lustrous black, surfacing, after immersion in the bath for an effective period of time.

As illustrative of the practise of my invention, stainless steel articles to be coated preferably are preliminarily cleansed such as by pickling, to eliminate substantially all scale, oxide film, grease, dirt or the like often present on the prod-- uct surfaces as a result of earlier treatment or fabrication. At times, I find advantage in resorting to mechanical cleansing methods such as sandblasting or grinding, with or without pickling. In pickling the stainless steel products, quick dipping in an aqueous solution of 20% nitiic acid. and 1% hydrofluoric acid usually is adequate. Thereafter, I prefer to rinse the products in clean Water, finally obtaining a passivated, scale-free and otherwise clean metal surface which usually is of dull gray or satin-like a pearance. then are ready for coating especially Where a dull blacl; finish is desired.

In the production of products-having a lustrous black finish, I prefer to introduce one or more polishing operations as after pickling but before black coating trea 'nent. A satisfactory polish at times is accomplished in this connection by buifing or by other mechanical methods. I find, however, that a better polish is achieved electrolytically as for example inaccordance with the rocess described and claimed in the recent United States Patent 2,335,354 of James N. O strofslry, entitled Polishing Stainless Iron and Steel, that is by making the products the anode of an aqueous electrolyte comprising an aliphatic-carboxylie acid such ascitric acid and a sol-- uble compound having a sulphate radical such as sulphuric acid; or by anodio treatment of the same in an aqueous solut on of concentrated perchloric acid as covered copending application, Serial to. 319, 35? entitled Electrolytic Polishing of Stainless Iron and Steel, now abandoned; or by subjecting the products to alternating current treatment in a concentrated acid bath including a substantial amount of nitric acid as set forth in the copending application Serial No. iGOjZQG entitled Electrolytic Polishing of Stainless Steel, now U. S. Patent 2,442,591, date-d June 1, 1948. After electrolytically polishing the steel products, I wash them in clean water so as to remove traces of the electrolyte employed. The products are ready for coating treatment.

It is conveniem to provide my treating bath for coating stainless steel articles and products, or other products as of carbon steel, in a melting vat or the like, equipped with a suitable heating unit. The bath itself consists of or comprises substantial amounts of sodium dichromate and/or potassium dichromate. In preparation and use constituents -ntageously are heated; at temperature substantially below decomposition temperature of the final melt. A bath con- The products subsequently are dried and taming either substantially all sodium dichromate or a predominance thereof is preferable, for this salt displays strong oxidizing eifects. Additions of somewhat less active potassium dichromate to the bath on the other hand afford the advantage of higher bath decomposition temperatures.

As indicated hereinbefore I employ a coating treatment which requires the circulation of oxygen illustratively in the form of air or commercial oxygen through the molten salt bath. The oxygen supply conveniently initiates from a pressure tank or gas pump and thence is brought into the dichromate as by means of suitable tubing connecting as through the side or bottom of the bath vat or container.

Before commencing the coating operations I find it desirable to adjust the bath to full treating temperature, usually to within the approximate range of 615 F. to 850 F., or even up to about 930 F. Preferably the temperature is maintained between 730 F. and approximately 800 F; Atthese temperatures, the bath is capable of exerting a. highly oxidizing effect and is ready for receiving the product to be coated.

I immerse a stainless steel product, for exam-= ple, in the coating bath for a period of time usually ranging from about two to thirty minutes or more. The treating bath temperature preferably is within the range of treating temperatures throughout the immersion perio'cLand the oxygen supply advantageously is continuous from beginning to end of the immersion treatment. In bubbling oxygen through the molten dichromate a limit on gas supply usually is imposed only by such factors as undue splashing set up by the rising bubbles in the instance of employing an open vat, and by bath cooling effect of the gas. I find. it convenient to leave the product immersed in the bath: as on a rack or in a holder throughout the entire treatment, or to immerse the product repeatedly as by dipping. At times at least I find advantage in extending the immersion time sufficiently to enable the product to assume a temperature equal to or near that of the moltenbath. In any event, the blackening bath" exerts an extremely powerful oxidizing actionon the immersed product and rapidly causes a black coating or film to' form on the metal body, which surfacing,- in the instance of stainless steeL is rich in oxides of iron and chromium. After treatment, I withdraw the coated stain less product or other metal article from the blackening bath, Wash the coated surface and cool the metal to room temperature. The resulting stainless steel product has the many beneficial characteristics of stainless steel such as its corrosion" resistance, hardness and strength, yet possesses a durable and beautiful black oxide finish which in no manner impairs the rustless or corrosionresisting properties of the underlying metal.

Articles blackened in accordance with my treatment possess a black coating which generally is more intense than those treated in a still bath; the coating usually displays an intense black hue throughout, and interference colors due to' refraction of light on or through the coatings obtained are at a minimum; During treatment thebubbling gas agitates the bath and is thought to' afford oxygen which contributes in making chemical blackening action on the immersed metal more positive and more complete, although I do; not wish to be bound by this explanation. The stirring effect achieved by the gas is of special value'irr the processing of large quantities of articles, particularly small intricately shaped pieces which when piled together inhibit a ready flow of chemical to all their parts. By elimination of agitatin mechanisms and the labor required to agitate products such as small parts, which usually is permissible with my process, the coating operations are simple and all the more economical.

The black oxide film or coating which I provide is dense, hard, tough and corrosion resistant and thus does not wear away or readily wipe off and, moreover, is highly adherent and flexible and does not crack, chip or spall ofi even upon being subjected to flexing or bending stresses. The coating whether dull or lustrous black in appearance, depending upon initial finish of the metal before dichromate bath treatment, is uniform and of stable quality. In products which I achieve, the coatings often are relied upon for superior corrosion resistance as compared with the underlying metal.

Thus it will be seen that there is provided in this invention an art in which the various objects hereinbefore noted, together with many thoroughly practical advantages, are successfully achieved. It will be seen that the product is strong, durable and corrosion resistant, and that it is given an attractive black oxide finish in a direct and thoroughly reliable manner without in any way impairing the rustless and corrosion resistant characteristics of the metal. Moreover, it will be seen that in the practice of my process, highly efiective and readily available treating constituents are combined for the purpose at hand to give unusually satisfactory results.

I claim:

1. In the blackening of articles and products containing at least chromium, the art which includes, treating the articles or products in a molten salt bath essentially consisting of salts of the group consisting of sodium dichromate and potassium dichromate, and circulating gas containing free oxygen through the bath during such treatment.

2. In the blackening of stainless steel articles and products, the art which includes, immersing the stainless steel articles or products for a period ranging from about two to thirty minutes time in a molten salt bath consisting of salts of the group consisting of sodium dichromate and potassium dichromate at a temperature of 615 F. to 930 F., and circulating gas containing free oxygen through the bath during such immersion.

3. In the blackening of alloy steel articles and products containing at least 10% chromium, the art which includes, treating the alloy steel articles or products by immersing the same in a molten salt bath consisting of potassium dichromate in substantial amount and a predominating amount by weight of sodium dichromate, and circulating gas containing free oxygen through said bath during the immersion treatment.

4. In the blackening of alloy steel articles and products containing at least 10% chromium, the art which includes, polishing such articles or products, immersing the same in polished condition in a molten salt bath consisting of salts of the group consisting of sodium dichromate and potassium dichromate, and circulating gas containing free oxygen through said bath during the immersion treatment.

5. In the blackening of stainless steel articles and products, the art which includes electropolishing such articles or products, immersing the same in polished condition in a molten salt bath consisting of salts of the group consisting of sodium dichromate and potassium dichromate, and circulating oxygen gas through said bath during the immersion treatment.

6. In blackening stainless steel articles and products, the art which includes, treating the stainless steel articles or products by immersing the same in a molten salt bath essentially consisting of sodium dichromate at a temperature in the range of about 615 F. to 850 F., and hubbling oxygen gas through said bath during the immersion treatment.

7. In blackening stainless steel articles and products, the art which includes, treating the stainless steel articles or products by immersing the same in a molten salt bath essentially consisting of sodium dichromate at a temperature in the range of about 730 F. to 800 F., and bubbling oxygen gas through said bath during the immersion treatment.

IRVINE C. CLINGAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,541,778 Agricola June 16, 1925 2,394,899 Clingan Feb. 12, 1946 FOREIGN PATENTS Number Country Date 688,812 Germany Mar. 2, 1940 OTHER REFERENCES Irvine Clingan, Black Oxide Coatings on Stainless Steels, Metal Finishing, March 1944, pages 139-140. 

1. IN THE BLACKENING OF ARTICLES AND PRODUCTS CONTAINING AT LEAST 10% CHROMIUM, THE ART WHICH INCLUDES, TREATING THE ARTICLES OR PRODUCTS IN A MOLTEN SALT BATH ESSENTIALLY CONSISTING OF SALTS OF THE GROUP CONSISTING OF SODIUM DICHROMATE AND POTASSIUM DICHROMATE, AND CIRCULATING GAS CONTAINING FREE OXYGEN THROUGH THE BATH DURING SUCH TREATMENT. 